How to make Feathers for a Gladiator Helmet with Particle Hair?
I would use a separate mesh for the emission of the hair particles. This allows for more flexibility and ease of customizing the hair later.


Add something like this on top of the helmet to act as the hair emitter:



You might want to separate part of the helmet mesh to ensure that there are no gaps between the emitter mesh and the actual helmet object. You might even want to put the emitter a little bit inside the helmet, just to be extra safe.
Add a particle system and change the type to hair (1). Also disable emitter so that the emitter object doesn't appear in the render (2), and enable child particles (3).  I used interpolated child particles, but this depends a bit on what you want to do.
Note that Simple child particles can result in floating hairs if you are not careful, especially on curved surfaces.



Note that I also tweaked the child particle settings to add a bit of randomness, so that the hairs are not all perfectly straight.
Add a material to the emitter. I used a simple translucent material:


Back in the particle settings of the emitter object, tweak the thickness and scale of the hairs as desired. You'll have to play with these settings a bit to get a feel for how they affect the render, but it should be pretty intuitive for the most part (e.g. thicker hairs will make it so less light can get through, making the hair appear more dense/dark in the render)

I went with a more uniform hair thickness, only tapering the thickness a little bit towards the ends of the hairs:




Example .blend